The top-seeded Kerber converted on seven of 10 break point chances and saved five of six on her own serve in a 50-minute, 61 60 rout. If she beats Patricia Mayr-Achleitner in Friday's quarterfinals, she will be Istanbul's eighth and final singles qualifier.

Still with an outside shot of moving ahead of Kerber in the race to Istanbul is Sloane Stephens, who overcame a 5-2 first set deficit and two set points in her 76(1) 46 63 defeat of Andrea Petkovic. In her previous encounter with Petkovic, Stephens won 8-6 in the third set at Wimbledon earlier this year.

"It was definitely tough for a second round match," Stephens said of Thursday's clash. "I thought I played well. Whenever I play Andrea, we have good matches, so it was good to get through that."

Stephens touched on her recent rankings ascension, as she improved to a career-high on Monday.

"I was excited when I moved from 13 to 12," Stephens said. "Just trying to take it slow and keep working on my game and improving. There's a long ways to go, but I'm excited to keep going."

Next up for the second-seeded Stephens is Stefanie Voegele, whom she recently beat in Tokyo.

"I played her in Tokyo two weeks ago," Stephens said. "I'm looking forward to getting back on court for the quarterfinals."

Also reaching the quarterfinals was No.7 seed Dominika Cibulkova, a 60 61 winner over Katarzyna Piter in 52 minutes. A showdown with No.2 seed and two-time Linz champion Ana Ivanovic awaits the Slovak, who was runner-up here in her only previous appearance in 2011.

At the final Premier Mandatory event of the campaign, the China Open, Agnieszka Radwanska reclaimed the trophy she won five years earlier, while over in indoor halls of Europe, Dominika Cibulkova and Svetlana Kuznetsova sealed their spots at the WTA Finals.